Abstract: This is a p-n junction device and the device comprises: a substrate 10 composed of a semiconductor material; a heavily doped n type sub-collector layer 14 over the substrate; a n type collector layer 16 over the sub-collector layer; a heavily doped p type first base layer 18, over the collector layer; a p type second base layer 20, substantially thinner than the first base layer, over the first base layer, with the second base layer being less heavily doped than the first base layer; and a n type emitter layer 24 over the second base layer, whereby, the second base layer serves as a diffusion barrier between the base and the emitter. Other devices and methods are also disclosed.